In the field of vehicle emissions controls, it is well known that during certain operating states of the engine undesired combustion products such as oxides of nitrogen (“NOx”) may be minimized by introducing a portion of the exhaust gases leaving the engine's combustion chambers back into the engine's intake manifold. The recirculated exhaust gas dilutes the incoming fresh intake air, resulting in a mixture to the engine that provides two primary mechanisms for reducing NOx formation. The first mechanism is the mixture reducing the peak in-cylinder combustion temperatures where the exhaust gas acts as a heat sink. The second mechanism is the dilution of the fresh air stream, displacing some of the oxygen which would have otherwise been drawn into the combustion chamber. The lower oxygen content results in fewer constituent oxygen atoms that feed the creation of NOx and results in an overall reduction of NOx formation.
In conventional internal combustion engines, such as for example the engine 1 shown schematically in FIG. 1, an exhaust gas recirculation passage 2 is provided between an exhaust line 10 leading away from the engine's combustion chambers 3 to the engine's intake manifold 20. The exhaust gas recirculation line is often provided with a cooler 22 for cooling the portion of exhaust gas being recirculated into the intake manifold, and a flow control valve 23. The flow control valve 23 may be opened, closed and/or throttled to control the amount of exhaust gas being recirculated and thereby better match the engine's recirculated exhaust gas need to the current engine operating state. If the engine is equipped with a turbocharger 30, the exhaust gas recirculation passage 2 is typically provided downstream of the turbocharger's compressor section 31, intercooler 40 and/or any intake flow control device 50, and upstream of the turbocharger's turbine 32 and exhaust gas treatment devices 60.
A well known problem with exhaust gas recirculation systems is the tendency for recirculating exhaust gas flow from the exhaust to the intake manifold to decrease or even halt during certain engine operating conditions, i.e., when there exists an unfavorable pressure ratio between the exhaust and the intake lines, or low exhaust mass flow rate conditions are present. For example, in response to a sudden increase in engine power output demand, there may be too little exhaust gas flow available in the exhaust to supply the intake manifold with sufficient recirculated exhaust gas to match the sudden increase in oxygen and fuel being supplied to the engine's cylinders. In such situations, the lack of sufficient recirculated exhaust gas may result in an inability to adequately suppress NOx formation during the transient condition, and a corresponding potential to exceed NOx emissions requirements.
An additional problem some recirculating exhaust gas systems can experience is with the use of a turbocharger, in that the turbocharger may develop sufficient pressure in the intake manifold to effectively halt the flow of exhaust gas through the exhaust gas recirculation line to the intake, particularly during engine output demand transients.
Previous attempts to improve exhaust gas recirculation flow primarily have concentrated on building backpressure in the downstream exhaust piping, such as by at least partially closing a downstream exhaust brake valve located upstream or downstream of the turbine side of a turbocharger, or by using a costly variable geometry turbocharger whose vanes may be adjusted to reduce flow through the turbocharger and thus build backpressure. Such approaches increase the pressure differential across the exhaust gas recirculation line between the exhaust line and the intake manifold. However, even with the assistance of such exhaust line components, adequate exhaust gas recirculation flow to the intake manifold cannot be assured in many transient engine operating conditions.
In view of these and other problems in the prior art, it is an objective of the present invention to provide enhanced exhaust gas recirculation flow in all operating engine conditions, including in particular transient engine operating conditions. It is a further objective to provide improved mixing of recirculated exhaust gas and fresh air in these operating conditions.
These and other objectives are addressed by a novel arrangement of a venturi and pitot tube nozzle components which establishes conditions in which the Coandă effect may be employed to enhance exhaust gas recirculation flow under virtually any engine operating condition. These arrangements thereby help minimize the potential for exceeding emissions limitations during engine operating conditions, including transient conditions.
The Coandă effect is a phenomenon in which a fluid jet, such as a gas jet, flowing in the same type of fluid (e.g., a gas jet flowing in a gas, or a liquid jet flowing in a liquid) is deflected as it passes an adjacent convex surface, following the contour of the convex surface until the fluid jet flow separates from the surface. In effect, the high-velocity jet conforms to the convex surface because the fluid in which the high-velocity jet is flowing is not present between the high-velocity jet and the adjacent convex surface, i.e., the high-velocity jet's deflection “fills” the vacuum which would otherwise be created between the high-velocity jet and the adjacent convex surface. The deflection toward the convex surface is accompanied by a drop in pressure and an increase in fluid velocity in the vicinity of the adjacent surface, in accordance with Bernoulli's fluid flow equations.
The decrease in fluid pressure resulting from the Coandă effect may be used to augment a pressure differential and/or to increase a mass flow rate in another fluid. For example, in aircraft applications a wing's lift may be increased by discharging the exhaust gases from a jet engine over the top surface of a wing so that high velocity exhaust gas jet flow deflected along the convex wing upper surface experiences a decrease in pressure, effectively increasing the pressure differential between the lower and upper surfaces of a wing. In other applications, the decreased pressure and increased velocity in the high-velocity jet at the point at which it must follow an adjacent convex surface may be used to enhance the mass flow rate of an adjacent fluid stream by: (i) increasing the differential pressure between an upstream point at which the adjacent flow stream enters the flow channel and the point where the Coandă effect is being generated, and (ii) increasing the lateral dispersion of the adjacent fluid into the high-velocity fluid by physical entrainment in the high Coandă flow region. This high velocity entrainment also helps promote more thorough mixing of the fluid streams.
In an embodiment of the present invention, an exhaust gas recirculation line having the general shape of a pitot tube is located with an opening of the pitot tube axially aligned with the center of an outer tube of a venturi manifold though which relatively high pressure fresh air flows. Preferably, the outlet of the pitot tube is facing in the downstream flow direction and is axially located in the vicinity of an entrance of a necked-down region of the outer tube. The transition between a decreasing-diameter region of the outer tube and a necked-down, relatively-straight region of the outer tube provides a convex wall surface on the inside of the outer tube. As the fresh air flowing in this area conforms to the convex surface, the Coandă effect results in decreased pressure and increased velocity in the fresh air. Further, by locating the outlet of the pitot tube at the outer tube center line in the transition region between the widest and narrowest regions of the outer tube (i.e., in the converging flow region), the outer surface of the pitot tube effectively reduces the cross-sectional flow area for the fresh air in the outer tube, thereby further increasing the fresh air velocity and lowering the fresh air pressure for a given fresh air mass flow rate.
The relationships between the parameters of the present invention's flow enhancement may be varied as needed for the application, as long as suitable flow performance is maintained. For example, in the prior art the “velocity ratio” between the velocity of the recirculating exhaust gas being injected and the fresh intake air upstream of the exhaust gas injection point had typically been on the order of approximately 1.5. With the enhancement associated with the application of the Coandă effect in the present invention, velocity ratios of approximately 2.0 to 5.0 result, allowing better mixing efficiency, increased exhaust gas flow and prevention of back flow in the exhaust gas conduit.
The parameters affecting the velocity ratio include inner diameter D1 of the fresh air tube upstream of the recirculating exhaust gas injection point, the inner diameter D2 of the narrowed portion of the tube, the angle of convergence a of the converging portion of the tube (D1, D2 and a thereby defining the length l of the converging portion), the depth of insertion x and the outer diameter d1 of the exhaust gas injection conduit into the converging portion of the tube, and the mass flow rate of the recirculating exhaust gas m1 and the fresh air m2. Taken together, these variables define annular area A between the injection end of the recirculating exhaust gas conduit and the laterally adjacent converging tube wall (i.e., A=f(D1, D2, d1, x, a), and the resulting velocity ratio VR (i.e., VR=f(D1, D2, d1, x, a, m1, m2).
With this arrangement of the exhaust gas recirculation pitot tube and the converging fresh air outer tube, the significantly reduced pressure in the vicinity of the outlet of the pitot tube presents a substantially enhanced pressure differential between the exhaust gas line and fresh air outer tube. This increased pressure difference serves to significantly increase the mass flow rate and velocity of the exhaust gas extracted from the pitot tube outlet, even during certain engine operating conditions in which the pressure ratio between the exhaust and intake manifolds is typically unfavorable.
A further benefit of the use of the Coandă effect is to enhance the lateral migration of the exhaust gas extracted from the pitot tube, where the decrease in pressure in the fresh air as it flows long the convex outer wall surface effectively draws the exhaust gas laterally toward the tube wall. This effect provides a much greater homogeneity in the recirculated exhaust gas fresh air mixture flowing toward the combustion chambers, and a corresponding cross-sectional velocity profile in the outer tube in which the typical “V” distribution from the center outward is flattened. The enhanced mixing also occurs in a much shorter distance than in conventional exhaust gas recirculation systems which simply discharge exhaust gas directly into the engine fresh air intake line. The lateral mixing of the exhaust gas with fresh air may be further enhanced by providing a divergent outer tube region downstream of the outer tube convergent flow region.
The improved homogeneity of the fresh air/recirculated exhaust gas mixture entering the engine's combustion chambers further enhances control of NOx formation in the combustion chamber by substantially reducing localized regions of over- and under-supply of recirculated exhaust gas in the combustion chamber.
Previous systems using a pitot tube to inject exhaust gas into the fresh air charge tube used a straight pitot tube in a manner which primarily only provided an exhaust gas entry point in the fresh air flow at or near a venturi section, resulting in the exhaust gas being swept along in a relatively homogeneous flow inside the fresh air column for a substantial downstream distance. Accordingly, in order to try to avoid an inhomogeneous air/exhaust gas mixture entering the engine cylinders (potentially increasing NOx emissions), previous designs required an undesirably long intake tract downstream of the exhaust gas injection point in order to provide sufficient gas mixing. Such long intakes are difficult for designers to accommodate in the highly space-constrained engine compartments of modern vehicles.
In order to address the lack of homogeneous mixture in the above-described previous design, it has been known to provide multiple small exhaust gas inlet tubes clustered in the fresh air charge tube to provide enhanced mixing at a shorter distance from the exhaust gas injection point. However, these designs suffer from the disadvantage of not providing a pressure gradient sufficiently high to ensure adequate exhaust gas recirculation flow in all engine operating conditions.
In contrast to these previous designs, the arrangement of the venturi and pitot tube to generate a Coandă effect-driven enhancement of exhaust gas extraction from the pitot tube outlet results in significantly greater homogeneity in the mixed fresh air/exhaust gas flow, and achieves a higher level of this homogeneity in a considerably shorter distance. This higher performance allows the designer much greater flexibility in engine systems layout, as excessively long downstream intake passages are no longer required to ensure a sufficiently well mixed charge arrives in the engine cylinders.
Variations on the above embodiment to achieve equivalent functional arrangements are possible. For example, rather than providing an outer tube which has to be contoured to achieve the converging, convex and diverging regions, an appropriately-shaped tubular insert may be placed inside a constant-section outer tube.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.